1. Field of Invention
This invention relates to stringed musical instruments, specifically to an improved apparatus for adjusting the curvature of the neck of a stringed musical instrument.
2. Prior Art
A stringed musical instrument needs a certain amount of curvature in the neck in order for the instrument to be properly playable. Due to variations in materials and design of the instrument's neck, the string tension alone does not always provide the right amount of neck curvature. A dual-acting truss rod assembly, embedded in the neck, is typically utilized to provide corrective adjustment, either forward or backward, of the curvature of the neck.
A common truss rod design is described in U.S. Pat. No. 6,259,008 to Eddinger (2001). In this design, opposing threads at opposite ends of the truss rod are utilized to allow adjustment in either a forward or backward direction. While this design does provide the required movement in both directions, the two threads working in opposite directions cause the truss rod to react very quickly to a given amount of rotation of the adjuster nut. The result is a truss rod that requires undue amounts of force to adjust and which is difficult to adjust precisely. Limitations in the strength of very fine threads make it impossible to achieve finer adjustment with this truss rod design.
U.S. Pat. No. 4,508,003 to Smakula (1985) describes a double acting truss rod which utilizes two differing thread pitches concentrically arranged on an adjuster bolt. This design offers dual direction adjustability and much finer adjustment capabilities. However, this thread arrangement requires considerable bulk resulting in a truss rod that is very large at the adjusting end. This limits the usefulness of this design to instruments with bulky necks or which allow the adjuster to operate from the end of the guitar neck attached to the body of the instrument. Many instruments do not meet these requirements thereby limiting the application of this design. In addition the adjuster mechanism is unduly complicated and costly to manufacture.